High-field ESR in the metallic fullerides RbC60 and CsC60

We discuss field-dependent electron spin resonance (ESR) experiments on the orthorhombic phase of RbC₆₀ and CsC₆₀. X-and W-band measurements have been performed to determine electronic scattering rates in the metallic temperature regime from 50 to 350 K. The insulating low-temperature phase (T < 50 K) has been investigated by X-, Q-, and W-band ESR, i.e., at 9, 34 and 95 GHz, in order to clarify whether collective resonance modes of a possibly magnetic ground state can be observed.     

Muon spin relaxation measurements of spin-correlation decay in spin-glassAgMn

The field (H) dependence of the muon longitudinal spin-lattice relaxation rate well below the spin glass temperature inAgMn is found to obey an algebraic form given by (H) ^v –1 withv=0.54±0.05. This suggests that Mn spin correlations decay with time ast ^–v , in agreement with mean field theories of spin-glass dynamics which yieldv0.5. Near the glass temperature the agreement between the data and theory is not as good.We are grateful to C.E. Olsen for preparing the samples, C. Boekema, S.A. Dodds, G.A. Gist, R.L. Hutson, S.L. Rudaz and A. Yaouanc for help during the experiments and to D.L. Huber and R.E. Walstedt for useful discussions. This work was performed under the auspices of the U.S. Department of Energy and was also supported by the U.S. National Science Foundation, Grant No. DMR-8115543.     

Muon spin relaxation in ferromagneticPdMn

Positive-muon ( ⁺) spin relaxation experiments have been carried out in the dilute ferromagnetic alloy Pd+2 at.% Mn (T _c=5.8 K). In the paramagnetic state the inhomogeneous ⁺ linewidth is proportional to the bulk magnetization. BelowT _c the ⁺ linewidth and the width of the ⁺ local field distribution in zero applied field are both in qualitative accord with the Sherrington-Kirkpatrick theory of disordered magnets.This work was performed under the auspices of the U.S. Department of Energy, and was supported by the U.S. National Science foundation, grant nos. DMR-7909223 and DMR-8115543, and by the Netherlands Stichting voor Fundamenteel Onderzoek der Materie (FOM).     

Muon spin relaxation in solid3He

Muon spin relaxation in solid³He depends non-monotonously on the temperature. It is shown that this is entirely due to the magnetic dipole-dipole interactions. The observed line narrowing cannot be explained by the mechanism of positive charge hopping described by an Arrhenius-like law. The relaxation rate at low temperatures was found to increase under the influence of an external electric field.     

Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt₃. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingT _c. Our results provide evidence that the lower superconducting phase in theH-T phase diagram of UPt₃ is characterised by broken time-reversal symmetry. Measurements of URu₂Si₂ and CeCu_2.2Si₂ indicate that the magnetically ordered volume fraction is temperature dependent in both systems.     

Muon spin relaxation in spin gap state of BaVS3

Positive muon spin relaxation measurements were performed on a spin-1/2 system BaVS₃, which shows a metal-insulator transition at T_MI= 70 K. We found a marked muon-spin depolarization below T_X= 30 K without appreciable critical divergence. The possibility of muonium formation in the insulating state rather than electron spin freezing is discussed taking into account the quenching of V spins evidenced by ⁵¹V NMR and NQR measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon spin relaxation and superconducting critical currents

Effect of pinning on muon spin relaxation is discussed. The application of μSR to study of the second type superconductors are considered.     

87Rb NMR investigations on binary and ternary fullerene compounds: Rb3C60, K2RbC60 and Rb2CsC60

Results of¹³C MAS NMR and⁸⁷Rb NMR measurements on Rb₃C₆₀, K₂RbC₆₀ and Rb₂CsC₆₀ are presented. The⁸⁷Rb NMR spectra show generally the same characteristics as already reported by other authors. For the ternary compound the degree of site preferential order of the alkali metal ions agrees with the analysis of X-ray absorption data. Furthermore, in the room temperature⁸⁷Rb NMR spectra of all samples an additional broadened line is detected indicating that at least for the samples prepared from sublimed C₆₀ a considerable amount of the Rb ions is located on sites with symmetry lower than cubic.     

Muon spin relaxation in different samples of DyAl2

Two samples of DyAl₂ were studied by longitudial μSR in the paramagnetic and ferromagnetic regime. In a region T_c±40 K the samples give different signals although X-ray analysis can not detect any impurity phase in either of them.     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

Muon spin relaxation in diluted triangular‐lattice antiferromagnet

Zero and longitudinal field μSR measurements on diluted triangular‐lattice antiferromagnet LuFeMgO₄ have revealed gradual and highly anisotropic slowing‐down of spin fluctuation. Relaxation rate \lambda in the condition with initial muon spins perpendicular to the hexagonal c‐plane shows divergent behavior while that in the parallel configuration remains finite. At certain temperature range the fluctuation of spins is suppressed by a small external magnetic field. Monte Carlo calculation suggests that the correlation time for z(c)‐component of spins diverges first on cooling. This revised version was published online in July 2006 with corrections to the Cover Date.     

Muon spin relaxation studies of magnetic-field-induced effects in high-Tc superconductors

Muon spin relaxation measurements in high transverse magnetic fields [FORMULA: SEE TEXT] revealed strong field-induced quasistatic magnetism in the underdoped and Eu-doped (La,Sr)2CuO4 and La1.875Ba0.125CuO4, existing well above Tc and TN. The susceptibility counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards T approximately 25 K. No field-induced magnetism was detected in overdoped La1.81Sr0.19CuO4, optimally doped Bi2212, and Zn-doped YBa2Cu3O7.     

Muon spin relaxation studies of extremely concentrated paramagnetic electrolyte solutions

The diamagnetic signal in aqueous solutions of non-reacting salts arises from muons substituted for protons in water molecules. For concentrated solutions of paramagnetic salts, muon spin relaxation results from the strong dipolar and scalar interactions between the paramagnetic ion and the muon. Information on dynamic and structural aspects of such solutions is derived from measurements of the transverse relaxation rate as a function of applied field over a wide range. Data are presented for concentrated solutions of Mn(NO₃)₂ and solutions of composition Mn _x Ca_(1–x (NO₃)₂·6H₂O.     

Muon spin relaxation by electronic excitations moving in one dimension

The manner in which an electronic spin, executing a linear random walk, e.g. along a polymer chain, depolarizes a muon (or proton) probe spin, is investigated by computer simulation. The essential features of the model are the assumptions of a contact hyperfine interaction with limited range and of loss of coherence between successive encounters. The low dimensionality of the motion is reflected in the shape of the relaxation functions generated, which depart significantly from simple exponentials. Fits to various functional forms are examined for different combinations of hop rate and chain length, hyperfine constant and applied magnetic field. This revised version was published online in August 2006 with corrections to the Cover Date.